Prior art optical recording media are known in which information may be written, but not erased. These "write-once" media have a thin film recording layer of TeO.sub.x (0&lt;x&lt;2.0) in which Te and TeO.sub.2 are the main compositional constituents.
The prior art also shows efforts directed towards the development of erasable optical recording media, in which it is possible to repeatedly write and erase information. Erasable recording media are being developed in which a small spot on a recording layer may be heated and melted by a focused laser beam. Rapid cooling transforms the molten spot into a non-crystalline amorphous material, having optical properties which are different from a crystalline state of the material. The same spot may subsequently be heated by the laser beam to a temperature which causes the amorphous state to be converted to the crystalline state. Information may generally be recorded by either forming the amorphous state or the crystalline state, with erasure accomplished by converting the recorded spots on the layer to the opposite state.
Materials which have been investigated in the prior art for such erasable recording media include thin film compositions of the chalcogen elements, as exemplified by Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2, etc., as reported by Ovshinsky et al. In addition, thin film recording layers formed from combinations of a chalcogen/elements with an element or elements selected from Group V of the periodic table or an element or elements selected from Group IV of the periodic table (i.e., Ge, As.sub.2 S.sub.3, As.sub.3 Se.sub.3 or Sb.sub.2 Se.sub.3) have also been widely investigated in the prior art.
Generally, an optical recording medium is in the shape of a circular disc, which is rotated during operation so that a movable laser beam can be rapidly focused over the entire disc surface. The thin film recording layer is deposited, along with other layers, on a transparent substrate in which grooves are formed to serve as guides for the laser light. The thin film recording layer may initially be prepared in the crystallized state.
To record information, laser light is focused to a spot on the recording layer of about one micron in diameter and is intensity-modulated between a first high peak power level and a second lower power bias level in accordance with the information to be recorded. Recording is performed while the disc is rotating by irradiating spots on the disc with a high peak power level, which is sufficient to increase the temperature of those spots above the melting point of the thin film recording layer. As these spots rapidly cool, the information is recorded by the formation of substantially non-crystalline, or amorphous spots in the recording layer.
To erase the information, the amorphous spots are irradiated with the lower power bias level of the laser light. Upon irradiation with this bias power level, the irradiated areas are elevated in temperature above the crystallization temperature of the thin film recording layer. Amorphous spots are thereby converted to substantially crystalline spots, and the information recorded therein is accordingly erased.
By utilizing a single laser beam modulated between a high peak power level and a lower power bias level, in combination with a recording layer which may be controllably converted between a substantially crystalline structure and a substantially non-crystalline or amorphous structure in the manner described above, an optical recording medium is produced in which it is possible to overwrite information in a simple procedure.
Since the thin film recording layer, during operation, is repeatedly heated, the optical recording medium is generally fabricated by sandwiching the thin film recording layer between two protective dielectric layers, which have the property of being highly heat resistant. These protective dielectric layers serve to thermally insulate the heated thin film recording layer from other thermally sensitive layers, such as the substrate and various adhesive layers. The thermal response, and in particular the rapid or slow cooling characteristics of the recording layer depend upon the thermal conductivity of the dielectric layers. Thus, by carefully selecting both the composition and geometry of these dielectric layers, it is possible to control and optimize the write/erase characteristics of the medium.
In the development of practical optical recording media, it is important to maintain the stability of the record/erase characteristics as a function of many write/erase cycles. After repeated cycling, deterioration of these characteristics may result from thermal damage to the disc substrate or other layers. This is manifested during operation as an increase in noise. In addition, a physical shifting of the thin film recording layer within the guide grooves of the substrate, generally along the direction of rotation of the disc, has also been identified as a factor leading to the deterioration of the write/erase characteristics after many cycles. The physical shifting of the layer also results from thermally induced stress.
As to the erasure characteristics, the melting point of a substantially non-crystalline or amorphous film containing Te is typically in a wide temperature range of 400 .degree. C. to 900.degree. C. As explained, crystallization is generally achieved by increasing the temperature of the irradiated spots above the crystallization temperature, followed by a gradual cooling. The peak temperature reached is within the crystallization temperature range, and is lower than the melting point of the thin film recording layer. However, when the crystallized layer is subsequently irradiated with laser light having a higher power level, it is heated to a temperature above the melting point of the material. As the molten area rapidly cools, the material transforms to the substantially non-crystalliine or amorphous state and an amorphous spot is thereby formed which may represent the recorded information. During formation of the amorphous spot, the faster the cooling rate, the more uniform will be the resulting amorphous state. If the cooling rate is too low, a difference in the degree of non-crystallinity between the center and the outer periphery of the spot may result, which can lead to incomplete or non-uniform erasure during subsequent write/erase cycles. Thus, the higher the cooling rate during formation of the amorphous state, the better the erasure characteristics of the optical recording medium.